The present invention relates to a method of producing a stator yoke of a small-sized motor and, more particularly, to a method of producing a stator yoke of the type having a multiplicity of teeth-like magnetic pole pieces (i.e., polar teeth) formed on its peripheral surface, e.g. the stator yoke of an inner rotor type stepper motor of small capacity.
The inner rotor type stepper motor of the kind described has been known. A typical example of this kind of motor will be outlined hereinunder.
Referring to FIG. 1, the stator portion is constituted mainly by an outer yoke 1, inner yoke 2 and a coil 3 disposed therebetween. Two units of the outer yoke 1, inner yoke 2 and the coil 3 are assembled to form the stator portion. The outer yoke 1 has a multiplicity of poplar teeth which are projecting in the axial direction from the inner peripheral portion thereof at a constant pitch. The inner yoke 2 has substantially similar construction to the outer yoke 1. With a coil 3 interposed therebetween, the outer yoke 1 and the inner yoke 2 are assembled together in such a manner that the polar teeth of the outer and inner yokes 1,2 are positioned in a staggered relation. A pair of assemblies, each having the inner and outer yokes, are assembled together with their inner yokes 2 positioned in a back-to-back relation and at an angular displacement that the polar teeth of the inner yokes 2 are offset or skewed at an angle amounting to one fourth (1/4) of the angular pitch of these teeth. Namely, the inner yokes 2 are in an opposed relation with each other, and the polar teeth of one inner yoke are staggered by one fourth of the angular degree of the polar teeth of the other inner yoke. This assembly accomodates a rotor portion which is rotatably mounted by means of the bearings 4. The rotor portion includes a shaft 5 and an annular permanent magnet 6 having a multiplicity of magnetized poles and fixed to the shaft 5. As an electric current is supplied in the form of pulses, the rotor makes an intermittent rotation in a stepped manner by the magnetomotive force.
Hitherto, some methods have been practically used for forming the stator yoke which has a multiplicity of inner peripheral polar teeth. For instance, a plurality of U-shaped cuts are formed radially in the end surface of a cup-shaped yoke, and the portions confined between the U-shaped cuts are bent inwardly to form a plurality of magnetic polar teeth. Alternatively, the end surface of the yoke shaped in a cup-like form is punched to have a form of an internally toothed gear and then the teeth portions are bent inwardly.
These methods, however, cannot provide polar teeth having sufficiently large length relative to the outside diameter of the motor and, therefore, cannot suitably be used in the production of motor having a large height and a large torque.
Particularly, in the former method, since the end surfaces of the magnetic polar teeth of one yoke oppose to the end surface, i.e. magnetic web of the other yoke, the magnetic flux inconveniently leaks to this portion to undesirably lower the torque. In addition, in the former method, a magnetic path between the polar teeth and the yoke body is constituted by the portions which are not punched out, it is not possible to excessively reduce the distance between the base portions of the adjacent magnetic poles.
In the latter method, the shape after the punching inevitably has a large distance between the base portions of the adjacent magnetic polar teeth.
Thus, neither of these known two processes can provide a large number of steps for a given outside diameter of the motor. In addition, in the conventional motors formed by these known processes, the magnetic pieces face in a flat plate-like manner to the cylindrical rotor surface, because these magnetic polar teeth are raised from the plane of the web portion of the yoke, so that a loss or waste of magnetic path is formed inevitably.